1. Wireless-Communication Networks Generally
Many people use mobile devices, such as cell phones and personal digital assistants (PDAs), to communicate with wireless-communication networks. Mobile devices and wireless-communication networks typically communicate with each other over a radio-frequency (RF) air interface according to a wireless-communication protocol. One example of such a wireless-communication protocol is Code Division Multiple Access (CDMA). Additional examples of wireless-communication protocols include WiMAX, LTE, IDEN, GSM, WIFI, and HDSPA.
Mobile devices typically conduct these communications with one or more base transceiver stations (BTSs), which send communications to and receive communications from mobile devices over the air interface via carrier signals. Each BTS is typically in turn communicatively connected with an entity known as a base station controller (BSC), which (a) controls one or more BTSs and (b) acts as a conduit between the BTS(s) and one or more switches or gateways, such as a mobile switching center (MSC) and/or a packet data serving node (PDSN), which may in turn interface with one or more signaling and/or transport networks. Thus, a mobile device may communicate with one or more remote devices via one or more such networks by sending communications, perhaps in the form of data packets, by way of a BTS. Communications from a BTS to a mobile device are typically referred to as “forward link” communications, whereas communications from a mobile device to a BTS are typically referred to as “reverse link” communications.
According to a particular type of CDMA protocol known as EV-DO, which stands for “Evolution Data Optimized,” networks provide service to mobile devices using a combination of time-division multiplexing (TDM) on the forward link and more-conventional CDMA technology on the reverse link. In the EV-DO context, a mobile device is typically referred to as an access terminal, while the network entity with which the access terminal communicates over the air interface is known as an access node. Such an access node may include a system of network devices, and typically includes a network entity known as a radio network controller (RNC), which is similar to a BSC.
In the balance of this written description, reference may be made to access terminals as examples of mobile devices, though this is for purposes of explanation and not to the exclusion of any other type of mobile device. Further, reference may be made to access nodes as examples of network entities with which an access terminal may communicate over an air interface, though this too is for explanation and not to the exclusion of any other type of network entity such as, but not limited to, a BTS. A particular access node with which an access terminal communicates may be referred to as a serving access node. Other access nodes in the wireless-communication network may be referred to as neighboring access nodes.
More generally, those of skill in the art will appreciate that aspects of this disclosure may be applied to various wireless-communication networks regardless of the particular configurations of such networks, and regardless of the particular wireless-communication protocols used by such networks.
2. Forward-Link Data Rate
As described above, communication between a serving access node and an access terminal may involve the serving access node sending packets to the access terminal on the forward link, and the access terminal sending packets to the serving access node on the reverse link. Focusing on the forward link, serving access nodes are generally able to transmit packets to access terminals at a number of different forward-link data rates. The particular forward-link data rates used by a serving access node may vary depending on, for example, the particular wireless-communication protocol according to which the access node operates. In the context of EV-DO, available forward-link data rates may be specified (i.e., represented) by—or at least associated with—particular data-rate control (DRC) values.
In some arrangements, an access terminal may be capable of requesting that a serving access node use a particular forward-link data rate for sending communications to the access terminal. In such arrangements, the access terminal may select a requested forward-link data rate based at least in part on an estimated carrier-to-interference (C/I) ratio of a signal received from the serving access node.
Generally, a C/I ratio provides an indication of the power of the signal received from the serving access node relative to the total power of interfering signals received from other transmitters, such as other access nodes in the wireless-communication network and/or any other transmitters, whether part of the same wireless-communication network or not. In typical arrangements, an access terminal may measure—at regular time intervals during a pre-determined recurring time period—the power of the signal received from the serving access node and the power of interfering signals received from, e.g., neighboring access nodes, and accordingly derive C/I measurements.
Such measurements may be used by the access terminal to estimate the C/I ratio of the signal emitted by the serving access node, as received by the access terminal. And it should be noted that the examples described herein that involve C/I ratios being determined in a manner where the “I” involves only signals from neighboring access nodes in the same network are used for clarity of presentation, and are not meant to imply that other types of interfering signals would not also or instead be relevant in other contexts.
In some arrangements, the pre-determined time period during which the access terminal makes C/I measurements may correspond to a coordinated “pilot-burst” time period, during which access nodes in the wireless-communication network emit a pilot signal. Such a pilot-burst time period may be specified by the particular wireless-communication protocol according to which the wireless-communication network operates.
Generally, pilot signals are used by access nodes to convey certain control, synchronization, and/or reference information to access terminals. However, pilot signals may also be used by access terminals for C/I estimation. According to some wireless-communication protocols, in particular some of those involving TDMA communications, access nodes transmit only pilot signals during the specified pilot-burst time period. Therefore, during the pilot-burst time period, C/I estimation may be carried out by the access terminal in the presence of only pilot signals emitted by the serving access node and neighboring access nodes. Accordingly, during such coordinated periods, access terminals may estimate C/I conditions in a relatively more rapid and/or accurate manner than may be possible, for example, during time periods of normal network communications.
An access terminal may be arranged to associate particular C/I ratios with respective particular forward-link data rates. Accordingly, the access terminal may be arranged to request a forward-link data rate associated with an estimated C/I ratio. For example, in situations where the estimated C/I ratio is relatively low, indicating relatively high interference conditions, the access terminal might request a relatively low forward-link data rate, typically better enabling the access terminal to demodulate signals received from the serving access node.
Generally, demodulation at a relatively low rate enables an access terminal to extract information of interest from the carrier signal in a manner that is less error-prone, i.e. necessitating fewer (and perhaps no) retransmissions of data, than demodulation at a relatively high rate, particularly in the presence of high-interference conditions. Conversely, and other things being (substantially) equal, in relatively low-interference conditions (high C/I ratio), an access terminal may request a relatively high forward-link data rate. On the one hand, it may be generally desirable for an access terminal to request a relatively high forward-link data rate, so as to receive data from the wireless-communication network at a high rate. On the other hand, however, use of a forward-link data rate that is unsuitably high for prevailing interference conditions typically tends to result in an increased incidence of errors in the demodulation of packets received by the access terminal, and thus a decrease in the effective rate of receiving data that—as a practical matter—is actually experienced by the access terminal.
As a general matter, it is quite typically the case that some of the packets received by an access terminal from an access node will contain errors, while some will not. A ratio can be computed between (i) the number of error-containing packets received by the access terminal (and perhaps a number of packets not received at all) from the access node over a given time period and (ii) the total number of packets received (or that should have been received) by the access terminal from the access node over that same time period. This ratio is known as the forward-link packet error rate (FPER). Generally, an access terminal may be arranged to request a forward-link data rate that corresponds to a desirable balance of data rate and error rate.